1. Technical Field of the Invention
The present invention relates to a tire for a vehicle wheel in which at least one of the beads comprises a seat having a generatrix, the axially inner end of which lies on a circle of a diameter greater than the diameter of the circle on which the axially outer end is located. This type of design is particularly suited to the new generations of tires which can be used, within certain limits, in conditions of low pressure, or even zero or virtually zero pressure, with the risk of separation of the tire from the rim on which it is mounted being eliminated. This concept is frequently designated by the expression “extended mobility”.
2. The Related Art
For a long time, tire manufacturers have been trying to develop a tire which does not create any source of potential risk or danger in the event of an abnormal drop in, or even total loss of, inflation pressure. One of the difficulties encountered relates to travelling with a flat tire or at very low pressure. In fact, when travelling at very low pressure, or even at zero pressure, with conventional tires, the beads are at great risk of separating from the periphery of the rim against which they are held by the inflation pressure. Numerous solutions have been tested in order to overcome these disadvantages. Frequently, these solutions cause additional difficulties in mounting and demounting the tire on/from the rim.
EP 0 582 196 (U.S. Pat. No. 5,660,656) discloses a tire comprising a tread extended by two sidewalls and two beads and also a carcass anchored in the two beads to an annular reinforcement. The carcass is formed of cords in an adjacent arrangement, which are aligned circumferentially and are in contact with at least one layer of bonding rubber of very high elasticity modulus in the hooking zone of the bead comprising the annular reinforcement. In this tire, the annular reinforcement of the hooking zone of the bead is formed of stacks of circumferential cords with the interposition of a layer of bonding rubber of very high elasticity modulus between the reinforcement cords of the carcass and the stacks of circumferential cords.
This embodiment is intended for tires of conventional type, with the beads being held against the rim hook due to the inflation pressure of the tire. In this type of arrangement, there is a predominance of stresses of the lateral or axial type, which induces major compressive forces that act substantially axially from the sidewalls towards the center of the bead. These forces increase according to the inflation pressure. The increase in pressure tends to make the bead slide against the hook, radially towards the outside. The stresses induced radially towards the inside, against the seat of the rim, decrease with an increase in pressure, or with any increase in the tension of the carcass-type reinforcement structure.
It will furthermore be noted that the stacks of cords are aligned in a direction substantially parallel to the orientation of the profile of the rim hook against which the bead bears.
The profile of the bead of this type of tire is relatively narrow and elongated; the anchoring is distributed over the major part of the height and width of the bead. The passage of the carcass into the bead is generally substantially central relative to the walls of the bead.
Furthermore, when a relatively narrow bead is subjected to predominantly axial stresses, neither the inflation pressure nor the tension induced in the carcass permits the generation of large moments or torques, which tend to make the bead pivot or turn on itself.
With such a type of tire, if the pressure drops and the vehicle continues to travel, the holding of the tire on the rim is no longer ensured, and in the majority of cases it rolls off the rim.
EP 0 673 324 (U.S. Pat. No. 5,634,993) describes a rolling assembly comprising at least one tire with a radial carcass reinforcement which is anchored within each bead and a rim of specific shaping. This rim comprises a first seat, with a generatrix such that the axially outer end of the generatrix is spaced from the axis of rotation by a length less than the spacing between its axially inner end and the axis of rotation, and is defined axially to the outside by a protrusion or rim flange. The tire comprises bead seats suitable for mounting on the rim. The type of tire/rim interface proposed in this document has many advantages compared with the solutions already known, in particular with regard to the ease of mounting/demounting, while making it possible to travel a certain distance despite a drop in pressure.
The present invention is directed in particular at proposing certain types of architecture in order to optimize the qualities of the assembly proposed in the document referred to above.
EP 0 748 287 (U.S. Pat. No. 6,179,028) describes a solution which permits initial optimization of the basic technology described in EP 0 673 324 referred to above. This is a tire, at least one bead of which has a structure that makes it possible to modify the clamping of the bead according to the tension of the carcass reinforcement and, in particular, reinforcement thereof when the inflation pressure increases to its rated value. The document thus proposes using a bead with anchoring of the end of the carcass by turning it up about the base of the bead wire, via the axially and radially inner sides relative to the bead wire. The bead also comprises, adjacent to the bead wire and axially to the outside thereof, a profiled element of rubber mix of relatively high hardness against which the bead wire can exert a compressive force when the tension of the carcass reinforcement increases. This compressive force creates self-lamping of the toe of the bead on the mounting rim. The tension of the carcass therefore involves displacement of the bead wire towards the outside, so that the latter generates the compressive force. In such a configuration, the presence of a bead wire of conventional type and the turning-up of the carcass beneath the latter are presented as being indispensable for generating the compressive force. This restricts the other types of arrangement which can be considered.
Moreover, EP 0 922 592 describes two embodiments with the carcass anchored by turning it up axially towards the outside. The first embodiment proposes anchoring of the carcass in the bead by turning it up radially towards the outside of the end of the carcass. The upturn is surrounded on either side by two radially superposed layers of metal wires arranged axially side by side and covering substantially all of the axial portion along the seat of the bead. The layers are arranged so as to be parallel to the seat. The types of cords and the corresponding dimensions are very precise.
The second solution proposed in this document relates to bead seats with different diameters. The securing of the carcass is also effected differently from the first solution. First of all, the carcass is subdivided into two portions which are radially separated at the level of the bead. Each portion is adjoined by a layer of cords which is arranged radially, each layer being arranged radially to the outside against each of the carcass portions. The radially outer carcass portion and the layer of cords radially to the inside are separated by an insert of the type of elastomer of high hardness provided in the bead. This insert axially lines the central portion of the bead and rises radially towards the outside and axially towards the inside, beyond the radial limit of the presence of the metal wires.
The two examples of solutions of EP 0 922 592 have several disadvantages. Thus, the securing of the carcass proposed in this document requires the presence of an upturn axially towards the outside of the end portion of the carcass. Furthermore, the superposed layers of cords are arranged radially close to the seat of the bead, for a good part at a radial position closer to the axis of rotation than the upper portion of the flange on which the bead bears. Unless highly extensible cords are used, it is difficult to mount/demount the tire, due to the unfavourable radial position of the cords. It will also be noted that the stacks are oriented substantially parallel to the profile of the seat against which the bead bears.
According to the second solution, the carcass is subdivided into two portions and an insert of high hardness is necessary to separate, on the one hand, the layers of cords and, on the other hand, the two carcass portions. However, the carcass is not anchored in the insert. The form of the insert described is limitative.